TERTIARY QUATERNARY EXTENSION-RELATED ALKALINE MAGMATISM IN WESTERN AND CENTRAL-EUROPE

Primitive mafic alkaline volcanic rocks from the Tertiary-Quaternary extension-related magmatic province of western and central Europe have major and trace element and Sr-Nd-Pb isotopic characteristics which suggest the involvement of both lithospheric and asthenospheric mantle source components in their petrogenesis. Oxygen isotope data for the volcanic rocks, mantle xenoliths, and lower-crustal mafic xenoliths indicate that extensive crustal contamination is not involved in the petrogenesis of these magmas. The geochemical characteristics of the lithospheric component are in part constrained by those of spinel lherzolite and mafic granulite xenoliths entrained within the magmas. This component appears to be the product of partial melting of phlogopite/amphibole-bearing mantle which was metasomatized as a consequence of magmatic activity during and preceding the Hercynian orogeny and during phases of Permo-Carboniferous extension. Partial melting of phlogopite is required to account for the generation of potassic magmas (leucitites and leucite nephelinites) with K2O/Na2O > 1. This component appears to differ between the individual Hercynian terrane blocks of Europe, reflecting, in part, their previous magmatic histories. The asthenospheric component has affinities with the source of St. Helena type (HIMU) OIB and may, in part, be 500-400 Ma recycled oceanic lithosphere subducted during the Hercynian orogeny. Alternatively, it could represent a zone of enriched mantle at the base of the subcontinental lithosphere which is preferentially partially melted during extension. There appears to be no need to invoke the existence of deep mantle plumes to explain the HIMU characteristics, although the data do not preclude them.